1. Field of the Invention
The invention relates to repeaters used in digital telephone communications, and more particularly, to an automatic gain control circuit for a digital baseband line equalizer such as that found in a T1 or an E1 repeater.
2. Related Art
In telephony, T1 and E1 are digital transmission standards. T1 is a standard used in the United States. A similar standard used in Europe is identified as the E1 standard. An important component of a digital transmission system is the repeater. Repeaters are placed at regular intervals along a transmission line in order to regenerate signals. A digital baseband line equalizer is invariably employed in the front end of repeaters such as those used in a T1 or E1 system. The equalizer filters and amplifies a signal which has been distorted during transmission over a length of cable.
The signal is a ternary digital signal transmitted at a rate of 1.544 Mbits/sec (megabits per second) in the T1 standard, and 2.04 Mbits/sec in the E1 standard. By ternary, it is meant that the signal has three logic levels: low, zero and high. Data is encoded using an alternate mark inversion (AMI) scheme in which a logical zero is transmitted as a zero voltage, and a logical one is transmitted as either a high or low voltage.
The equalizer used in the front end of these repeaters includes an automatic gain control (AGC) circuit which modifies the gain and frequency response of the equalizer to adjust for varying line conditions. It is intended for the AGC circuit to maintain a constant amplitude signal at the output of the equalizer. However, with conventional AGC circuits, the amplitude of the output signal tends to vary as a function of the data density of the signal. For example, if a data signal has a large number of zeros occurring one after the other, then it will appear to the conventional AGC circuit that the gain needs to be increased, even though a correct amplitude data stream is being received. Thus, the gain of the circuit tends to be dependent on the content or density of the data being received at the repeater.
What is needed is an equalizer that will produce a constant amplitude output signal independent of data density.